1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticule, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
The throughput of a lithographic apparatus is controlled by, amongst other things, the area of the substrate exposed per exposure of the apparatus. In a conventional lithographic apparatus, the throughput is determined by the number of exposure fields step-and-scanned over the substrate per unit of time. The exposure field size is determined by the reticule field size divided by the magnification of the projection system, typically 4×, but also, e.g. 5× or 6×.
The throughput of an apparatus comprising an array of individually controllable elements instead of a mask (e.g., a maskless lithographic apparatus) is determined by the number of pixels in the field multiplied by the area exposed per pixel divided by the number of passes used to print each pattern, typically 2 but also 3 or 4 passes are used. A larger number of passes enhances the dose control and smoothes out the non-uniformities induced by the pixel raster. For effective exposure of the substrate the pixel size must be approximately half the size of the minimum feature. Thus, for areas of a substrate in which the minimum feature size is larger than for the minimum feature size overall (i.e., the resolution is lower), the apparatus is not operating at maximum efficiency. Throughput is lower than is possible, leading to decreased revenue.
Therefore, what is needed is a system and method that increases efficiency of a maskless lithography system to increase throughput.